Rosuvastatin, which is an antihyperchlolesterolemic drug, is chemically known as (E)-7-[4-(4-fluorophenyl)-6-isopropyl-2-[methyl(methylsulfonyl)amino]pyrimidin-5-yl](3R,5S)-3,5-dihydroxyhept-6-enoic acid calcium (2:1) salt of Formula I.
Rosuvastatin was for the first time disclosed in U.S. Pat. No. 5,260,440. Rosuvastatin is being marketed under the proprietary name CRESTOR, as an oral tablet, for the treatment of hypercholesterolemia.
In view of the importance of rosuvastatin as a lipid-lowering agent, several synthetic methods have been reported in the literature to prepare rosuvastatin, some of which are summarized below:
U.S. Pat. No. 5,260,440 disclose a process for preparing rosuvastatin in examples. The process is as shown below:

The difficulties in the above process are that the intermediate (A) is not obtained in pure form readily. Further, its purification is tedious and overall yield is extremely low. Even when intermediate (A) is obtained in pure form, further condensation with intermediate (11) to form rosuvastatin, does not result in rosuvastatin of right quality as the product contains unacceptable quantity of impurity levels.
WO 03/097614 A2 describes a modified procedure for the preparation of the starting material 4-(4-fluorophenyl)-6-isopropyl-2-[methyl(methylsulfonyl)amino]pyrimidin-5-carboxaldehyde and its further conversion to rosuvastatin by condensing with methyl (3R)-3-[(tert-butyldimethylsilyl)oxy]-5-oxo-6-triphenylphosphoranylidene hexanoate. The condensed product was deprotected using methanesulfonic acid and subsequently converted to rosuvastatin calcium (2:1) salt.
WO 2004/052867 A1 describes a process to prepare rosuvastatin by condensing 1-cyano (2S)-2-[(tert-butyldimethylsilyl)oxy]-4-oxo-5-triphenylphosphoranylidenepentane with 4-(4-fluorophenyl)-6-isopropyl-2[methyl(methylsulfonyl)amino]pyrimidin-5-carbaldehyde and subsequent deprotection of silyl group, followed by reduction and hydrolysis.
WO 2000/049014 A1 discloses a novel chemical process for the manufacture of tert-butyl (E)-(6-{2-[4-(4-fluorophenyl)-6-isopropyl-2-methyl(methylsulfonyl)amino]pyrimidin-5-yl]vinyl}-(4R,6S)-2,2-dimethyl[1,3]dioxan-4-yl)acetate, which comprises reaction of diphenyl {4-(4-fluorophenyl)-6-isopropyl-2[methyl(methylsulfonyl)amino]pyrimidin-5-yl-methyl}phosphineoxide with tert-butyl 2-[(4R,6S)-6-formyl-2,2-dimethyl-1,3-dioxan-4-yl]acetate and its further conversion to rosuvastatin.
WO 2004/014872 A1 describes a process for the manufacture of rosuvastatin calcium (2:1) salt, which comprises mixing a solution of calcium chloride with a solution of water soluble salt of (E)-7-[4-(4-fluorophenyl)-6-isopropyl-2-[methyl(methylsulfonyl)amino]pyrimidin-5-yl](3R,5S)-3,5-dihydroxyhept-6-enoic acid. This process for the preparation of rosuvastatin employs the use of phosphorane side chain, the preparation of side chain requires eight synthetic steps and involves expensive reagents. The process is both uneconomical and time consuming, hence not appropriate for commercial scale operation.
WO 2006/100689 A1 discloses a process for preparation of rosuvastatin as shown below:

In the above scheme R1, R2, R3 represent substituted or unsubstituted phenyl and R4 represents an aliphatic residue selected from C1-C4 alkyl, R5 represents C1-C4 alkyl which is optionally substituted by hydroxyl, R6 represents hydrogen, halogen, C1-C4 alkyl or C1-C4 alkoxy, R7 represents aliphatic residue, R8 represents C1-C4 alkyl.
WO 2006/106526 A1 describes the preparation of rosuvastatin as shown below:

In the above mentioned scheme R1, R2, R3 are substituted or unsubstituted phenyl and R4 is an aliphatic residue selected from C1-C4 alkyl, R5 represents C1-C4 alkyl, M is an alkali metal salt,
X represents a halogen, R6 represents C1-C4 alkyl which is optionally substituted by hydroxyl, R7 represents hydrogen, halogen, C1-C4 alkyl or C1-C4 alkoxy, R8 is an aliphatic residue selected from C1-C4 alkyl.
As mentioned above though there are a number of processes available, still there is a continuing need to identify alternative processes for the manufacture of rosuvastatin and its pharmaceutically acceptable salts. Such processes may, for example, when compared to previously known processes, be more convenient to use, be more suitable for large scale manufacture, give the product in a better yield, reduce the number of steps involved, use intermediates which are more easily isolated, require less complex purification techniques, use less expensive reagents and/or be more environmentally friendly.
We have now found an improved process for preparing (E)-7-[4-(4-fluorophenyl)-6-isopropyl-2-[methyl(methylsulfonyl)amino]pyrimidin-5-yl](3R,5S)-3,5-dihydroxyhept-6-enoic acid calcium of Formula I